Digital tone generators for musical instruments are well known in which the waveshape of a tone is the same for each musical note. In other words, the harmonic contents of each musical note over the full range of the musical scale is substantially the same. The harmonic content can be modified to provide different waveshapes, as by setting "Stops". Such tone generation is particularly suited to electronic musical instruments that are intended to imitate the tonal characteristics of pipe organs. However, most other musical instruments have resonant characteristics which do not change with the pitch of the note being produced. Thus the tones for notes sounded up and down the scale do not exhibit constant harmonic structure. Rather the harmonic structure can be characterized by a signal whose spectral content is modified by passing the signal through a fixed filter. The use of fixed filters on the output signals of analog type tone generators is well known. The use of fixed filters on the analog output signals of a digital tone generator, of course, can be used to produce the same audio effects. However, the design of conventional analog filters to emulate the transfer characteristics of various acoustical instruments results in rather complex filter designs. Moreover, to make such filters variable so as to provide resonances at various frequencies becomes impractical for anything but the simplest transfer characteristics.
The use of digital techniques to provide fixed format filtering has heretofore been proposed for use in a computer organ. See, for example, U.S. Patent No. 3,956,960. In the computer organ described in this patent the amplitudes of a series of points defining the waveshape of the tone being generated are computed in real time by summing, for each point in time, the amplitudes of each of the Fourier components of the desired waveshape. Filtering is accomplished by applying a scale factor to the amplitude coefficient of each Fourier component, the scale factor being selected according to the frequency of the Fourier component. Selection of the scale factor for each Fourier component is derived from a frequency number R whose value is determined by the particular key operated on the keyboard of the instrument. Frequency information is essential in the computer organ during the computation of the amplitude values in real time. The same frequency information is therefore available to select the appropriate scale factor as a function of frequency to scale the Fourier components in conformance with the transfer characteristics of the fixed formant filter.
The present invention is directed to a fixed formant filter particularly suited for use in a polyphonic digital tone synthesizer of the type described in U.S. Pat. No. 4,085,644. In the polyphonic tone synthesizer, waveshape information is computed by calculating its constituent generalized Fourier components and summing the Fourier components to form a master data set corresponding to the amplitudes of a series of points defining the waveshape of one cycle (or half cycle) of the tone to be generated. The amplitude defining words of the master data set are then transferred sequentially at a rate determined by the fundamental frequency of the tone being generated to a digital-to-analog converter which converts the master data set to an analog voltage of the desired waveshape. Calculation of the master data list is entirely in the harmonic mode and is independent of the pitch of the note being generated. The present invention provides a means for scaling the Fourier components during calculation of the master data set such that the master data set conforms to a waveshape which is dependent on the pitch of the note being generated, i.e., a waveshape modified by a fixed formant filter. The invention is an improvement over the formant filter described in the above-identified U.S. Pat. No. 3,956,960 in that it does not require a frequency dependent R number. Moreover, the present invention provides a fixed formant filtering characteristic for a frequency modulated tone. Also, an interpolation feature allows a relatively small number of scale factors, less than twice the number of notes on the keyboard, to be used in producing relatively complex filter transfer characteristics.